1. Field of the Invention
This invention relates to sodium sulphur cells in which a solid electrolyte, typically beta alumina, separates molten sodium from a cathodic reactant comprising sulphur and sodium polysulphides.
2. Prior Art
In a sodium sulphur cell, as the cell discharges, sodium ions pass through the electrolyte into the cathodic reactant to combine with negatively-charged sulphide ions to form the sodium polysulphides. On charging, the reverse action takes place. It is necessary to inject and to extract electrons from the cathode electrode. The molten sulphur/polysulphides however has a very poor electrical conductivity and it is the practice therefore to provide a porous conductive body such as a fibrous graphite or carbon matrix in the cathodic region. This matrix extends through the cathodic region and enables electrons to be supplied to and removed from the cathode reactant. The matrix must be porous to allow for the physical movement of the molten reactant material. In order to provide a current connection from an external circuit into the cathodic region, a current collector has to be provided which is connectible to an external circuit and which is in electrical contact with the aforementioned matrix material so that electrons can flow from the current collector into the matrix or vice versa.
It is of particular importance to have a small resistance in batteries for use in vehicles where often a very large power has to be extracted for a short time. A large current can only be drawn from the cell efficiently if the resistance of the cell is small. Thus one of the requirements for a sodium sulphur cell is to have a cathode current collector having a very low electrical resistance.
A major problem however in the construction of a cathode current collector for a sodium sulphur cell arises because the sulphur/polysulphide materials forming the cathodic reactant are highly corrosive. It is for this reason that a carbon or graphite matrix is commonly employed in the cathodic region. Carbon is not attacked by the cathodic reactant and thus is commonly used to form a porous conductive matrix despite the fact that it has a relatively low electronic conductivity compared with many materials such as metals. Many cells are described in the literature employing stainless steel as a cathode current collector but the life of stainless steel in this cathodic reactant is short and such material is not considered acceptable except for laboratory type cells.
It is the common practice in sodium sulphur cells to make use of a tubular electrolyte, usually in the form of a tube closed at one end. In a tubular cell, the cathodic reactant may be put inside the electrolyte tube and the sodium around the outside of the tube within an outer housing. Such a cell is commonly referred to as a central sulphur cell. Alternatively the sodium may be put inside the electrolyte tube and the cathodic reactant around the outside of the electrolyte tube within an annular space between the electrolyte tube and the housing. Such a cell is commonly referred to as a central sodium cell. In a central sulphur cell, the cathode current collector conveniently is a rod or tube located axially within the electrolyte tube, the annular space between the current collector rod or tube and the electrolyte being filled with the matrix material impregnated with sulphur/polysulphides. Many such cells have been built with a solid carbon current collector rod. The conductivity of such a rod is not as good as would be desirable in order to make use of the high discharge currents which are possible with a sodium sulphur cell and, for this reason, a number of proposals have been made for current collector rods of more complex construction which would have a higher conductivity. For example in the U.S. Pat. Nos. 3,982,957, 4,061,840 and 4,066,826 there is described and claimed a construction in which a carbon or graphite tube is used with a core of much more highly conductive material e.g. aluminium and with a suitable deformable interface between the core and internal surface of the carbon or graphite tube to provide an electrically conductive path despite the different coefficients of thermal expansion of the core and tube. Such a current collector however requires that the carbon or graphite tube should be impermeable to the cathodic reactant and, whilst this can be achieved, it leads to a complex manufacturing operation and a relatively complex current collector construction. Other proposals have been made to coat a rod of material of high electrical conductivity with a conductive oxide or carbide coating or with a plurality of coatings in order to obtain the advantage of the high conductivity of the metal core whilst protecting that metal from any contact with the cathodic reactant. In such constructions, the integrity of the cell depends on the coating remaining intact to isolate the metal core from the cathodic reactant and this leads to relatively complex techniques to ensure proper coating.
In central sodium cells, the cathodic reactant is outside the electrolyte tube and it is the common practice to employ the housing as the cathode current collector. The housing is of relatively large surface area compared with the current collector in a central sulphur cell and hence the current density is smaller. It has been proposed for example to use a stainless steel housing with a protective coating of molybdenum or carbon. Once again however the integrity of the cell is dependent on the coating giving complete protection for the stainless steel.
It will be noted that in all these proposals for coated metal cathode current collectors, it is accepted that the coating must be of an electronically conductive material. This is because there must be an electrically conductive path between the metal substrate and the matrix material in the cathodic region of the cell.